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The rib cage, also known as the thoracic cage, is a cage-like structure of bones that frames the chest cavity. It consists of 24 ribs (12 on each side) that wrap around the torso from front to back. The diaphragm, which is the main muscle of inspiration, also makes up the floor of the thorax, separating the contents of the chest from the abdomen. The thoracic wall is made up of five muscles: the external intercostal muscles, internal intercostal muscles, innermost intercostal muscles, subcostalis, and transversus thoracis. The scalene muscles, sternocleidomastoid muscle, serratus posterior superior, and levatores costarum are also involved in rib elevation.
| Characteristics | Values |
|---|---|
| Number of bones in the rib cage | 37 |
| Number of ribs | 24 (12 on each side) |
| Number of intercostal muscles | 11 pairs |
| Muscles that elevate ribs | External intercostal muscles, serratus posterior superior, scalene muscles, sternocleidomastoid muscle |
| Muscles that depress ribs | Internal intercostal muscles, innermost intercostal muscles, subcostalis, serratus posterior inferior |
| Muscles that stabilise the lower ribs | Serratus posterior inferior, quadratus lumborum |
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What You'll Learn
External intercostal muscles
The external intercostal muscles are a set of muscles located between adjacent ribs, attaching along the shafts of the ribs and extending obliquely from the lower border of one rib to the upper border of the adjacent rib below. There are 11 pairs of external intercostal muscles on both sides of the body. They are the most superficial set of muscles that occupy the 11 intercostal spaces. Their name is derived from their spatial relationship with other intercostal muscles, as they are found superficially to the internal and innermost intercostals.
The external intercostal muscles are located superficially to the internal intercostal muscles, separated from them by a thin fascia. Superficial to the external intercostal muscles lie the pectoralis major and minor, serratus anterior and the upper part of the rectus abdominis muscle. Each intercostal muscle occupies its intercostal space, starting from the tubercles of ribs posteriorly to the costochondral junctions anteriorly. The external intercostal muscles receive innervation from intercostal nerves of corresponding intercostal spaces, which are the anterior rami of thoracic spinal nerves.
The external intercostal muscles are one of the accessory respiratory muscles. They elevate the ribs during forced inspiration, increasing the transverse and anteroposterior diameter of the lungs, which in turn decreases the intrapleural pressure. This process expands the lungs and facilitates the entry of air into them. During inspiration, the internal intercostal muscles relax while the external muscles contract, causing the expansion of the chest cavity and an influx of air into the lungs.
The external intercostal muscles work in conjunction with the internal and innermost intercostal muscles to maintain the stability of the thoracic cage. The internal intercostal muscles lie deep to the external intercostals and extend from the rib above to the one below, but in an opposite direction. They are continuous with the internal oblique muscle of the abdominal wall. The innermost intercostal muscles are the deepest of the intercostal muscles and are similar in structure to the internal intercostals. They are separated from the internal intercostals by the intercostal neurovascular bundle.
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Internal intercostal muscles
The internal intercostal muscles are a muscle of the thoracic wall, located in the intercostal spaces between the ribs. They are part of the middle layer of the intercostal muscle group, which also includes the external and innermost intercostals, serratus posterior, levatores costarum, subcostal, and transversus thoracis muscles. The internal intercostals assist in breathing and support the integrity of the thoracic cage.
There are 11 internal intercostal muscles, filling 11 intercostal spaces by extending between the inferior margin of the superior rib to the superior margin of the first rib below. The muscle fibres fill the intercostal space from the lateral border of the sternum to the posterior angle of the ribs onto which they attach. The internal intercostals originate from the inferior margin of costal cartilage and the inner lip of the costal groove. Their fibres course posteroinferiorly and medially to insert into the superior border of the immediate inferior rib.
The internal intercostals are located deep to the external intercostals, from which they are separated by a thin fascia. They lie superficially to the innermost intercostal muscles. The posterior surface of the superior margin of each internal intercostal encloses the intercostal tunnel with the anterior surface of the corresponding innermost intercostal. The intercostal nerve, artery, and vein traverse this tunnel.
The internal intercostal muscles are innervated by the intercostal nerves of corresponding intercostal spaces. They receive blood supply from the muscular branches of the anterior and posterior intercostal arteries. Venous blood is conveyed by the anterior and posterior intercostal veins to either the brachiocephalic or azygos venous system, which both empty into the superior vena cava.
During exhalation, the internal intercostal muscles depress and retract the ribs, compressing the thoracic cavity and expelling air. They are only used during forceful exhalation, such as coughing or during exercise, and not during relaxed breathing. During inspiration, the external intercostal muscles and the parasternal part of the internal intercostal muscles aid in elevating the ribs and expanding the thoracic cavity.
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Innermost intercostal muscles
The innermost intercostal muscles are the deepest layer of the intercostal muscles, which are the muscles of the thoracic wall. They are located deep to the internal and external intercostals, filling the 11 intercostal spaces between the ribs. They originate from the inner surface of the costal groove of the ribs, inserting into the superior surface of the rib below.
The fibres of the innermost intercostal muscles are oriented in an inferior and posterior direction. They are not equally developed in every intercostal space, with the least developed muscles located in the superior intercostal spaces, and the most developed in the inferior intercostal spaces. Even the most developed innermost intercostal muscles do not fill the entire length of the intercostal spaces, but rather fill the middle two quarters.
The precise function of the innermost intercostal muscles is not yet known, but it is likely to be the same as the internal intercostal muscles, which are similar in structure. It is suggested that they depress the ribs and aid in pushing air out of the lungs during forced expiration. This puts them into the category of accessory respiratory musculature, which is only active during forced breathing. They also support the integrity of the thoracic cage.
The innermost intercostal muscles are supplied by the vessels of the thoracic wall and innervated by the intercostal nerves, which are the anterior rami of the thoracic spinal nerves.
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Serratus posterior superior
The serratus posterior superior is an accessory muscle for respiration. It originates from the C7-T3 vertebrae spinous process and the nuchal ligament. It attaches to ribs 2 through 5 and elevates them during inspiration. The serratus posterior superior is innervated by the intercostal nerves T2-T5 and the ventral rami of the intercostal nerves of T1-T5.
The serratus posterior superior is located superficial to the thoracic part of the thoracolumbar fascia, covering the splenius cervicis. It lies deep to the rhomboids and the trapezius muscle. To expose the serratus posterior superior muscle, it is necessary to manoeuvre the shoulder blade.
The function of the serratus posterior muscles is still debated, but the general consensus is that they play a role in respiration. The serratus posterior superior elevates the ribs and supports inspiration. The disproportionate use of the serratus posterior superior and other back muscles, such as cradling the phone between the ear and shoulder, can result in scapulocostal syndrome. This condition is characterised by pain and paresthesia along the medial border of the scapula, radiating to the neck, chest, and upper extremities.
The thoracic wall is made up of five muscles: the external intercostal muscles, internal intercostal muscles, innermost intercostal muscles, subcostalis, and transversus thoracis. These muscles are responsible for changing the volume of the thoracic cavity during respiration. The diaphragm is another muscle in the thorax that serves as the main muscle of inspiration. It makes up the floor of the thorax, separating the contents of the chest from the abdomen.
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Sternocleidomastoid muscle
The sternocleidomastoid (SCM) muscle is a powerful neck muscle that allows you to bend your neck and turn or tilt your head. It is the largest muscle in the front of the neck and can be felt on both the right and left sides.
The SCM helps to stabilise your neck and maintain your posture. It also plays a role in breathing, working with other neck muscles to lift your breastbone and collarbone when you inhale, creating space for your lungs to take in air.
The SCM is involved in the movement of the temporomandibular joint (TMJ), which is the joint that connects your jaw to your skull, allowing you to open and close your mouth.
Injuries, tension, sprains, strains, atrophy, and tumours can all damage the SCM muscle, leading to pain, stiffness, and neck problems such as torticollis (wryneck). Treatment options include massage, osteopathic manipulation, physical therapy, and surgery in severe cases.
The SCM is innervated by the accessory nerve, which supplies motor fibres. The signalling process to contract or relax the SCM begins in the accessory nerve nucleus in the anterior horn of the spinal cord. Acetylcholine is released, initiating an action potential that travels along the muscle fibre, resulting in contraction.
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Frequently asked questions
The muscles that elevate the rib cage include the scalene muscles, sternocleidomastoid muscle, serratus posterior superior, levatores costarum, and the diaphragm.
The intercostal muscles are the muscles between two or more ribs. There are three types of intercostal muscles: external, internal, and innermost intercostal muscles.
The intercostal muscles help to stabilise the position of the ribs during normal respiration. They also assist in breathing and help to build the chest wall.
